The present invention relates to a method and apparatus for the synchronization of the data in a telecommunications system and downconversion of data modulated onto a radio frequency carrier.
In the area of telecommunications, there have been advances in Fiber-to-the-Curb (FTTC) technology in which devices in the home are connected to the telephone central office via a network of one or more optical fibers connecting the central office to network points called Broadband Network Units (BNUs) which in turn connect to the subscriber home via a coaxial cable, and to the devices in the home via a passive splitter and in-home coaxial wiring. In these FTTC networks, signals can be routed to the home via a single coaxial cable connecting the home to the BNU.
Using the FTTC configuration with a coaxial drop cable, devices in the home including telephones, computers and televisions will be connected to the coaxial network. Because there are multiple devices in the home which will all transmit back to a central receiver, and because telecommunications services require accurate clocks for data recovery and transmission, it is important to have a method of synchronization which allows the devices in the home to recover an accurate system clock, and to transmit and receive on the correct frequencies and at the correct data rates.
In addition to the above mentioned problem, there are multiple devices in the home which are connected to the BNU on a single coaxial cable, a multiple access protocol such as Time Division Multiple Access, Code Division Multiple Access, or Frequency Division Multiple Access is used to access the BNU from the devices in the homes. These multiple access protocols typically require an accurate bit clock as well as an accurate carrier frequency.
When data is received at the BNU, it is necessary to process the signal, which is typically a radio frequency signal, and recover the information transmitted from the devices. A first step in recovering the information is downconversion, in which the radio frequency signal is shifted in frequency to a baseband signal. This process can be complex and can require an excessive amount of signal processing power due to the high data rates and the high speed sampling required to digitize the radio frequency signal.
Another difficulty which is encountered in such a system is the fact that other services such as traditional cable TV may be provided on the same coaxial drop cable, and thus it is important to select frequencies for both downstream (BNU-device) and upstream (device-BNU) transmission which do not interfere with the cable TV services which are typically in the 50-550 MHz frequency range.
For the aforementioned reasons there is a need for an accurate, low cost method of synchronization in a FTTC system which employs a coaxial drop to the home, and to select frequencies for downstream and upstream transmission which permit use of the coaxial cable for the transport of other services at other frequencies. It is also necessary to have an efficient method of downconversion.
In a FTTC system transmission of downstream data from a BNU to devices in a residence is accomplished by transmitting downstream data on a downstream radio frequency (RF) carrier in which both the downstream data rate and downstream RF carrier are integer multiples of a sub-harmonic of a master clock.
Data from devices in the residence is transmitted to the BNU on an upstream RF carrier, and both the upstream RF carrier and upstream data rate are integer multiples of a sub-harmonic of a master clock.
In one embodiment, a 155.52 Mb/s data signal is received at the BNU over an optical fiber and within the BIU (in the BNU) the clock recovered from this signal serves as a reference clock, from which a 19.44 MHz clock (xe2x85x9 of the 155 Mb/s clock signal) is produced. A local oscillator at 622.08 MHz is locked to the 19.44 MHz clock. The 622.08 MHz clock serves as a carrier for the downstream data, which has a data rate of 51.84 Mb/s. A 51.84 MHz clock can be easily derived from the 155.52 MHz clock. A 38.88 MHz receive clock is also generated at the BIU, based on locking to the 19.44 MHz clock.
At the NIM (in the device in the home) the 622.08 MHz carrier is recovered, as is the 51.84 MHz bit clock. Because these two signals are synchronous, a number of methods can be used to derive them. A 19.44 MHz clock, 77.76 MHz clock, and 38.88 MHz clock can all be readily generated, since the system is synchronous and all of the frequencies are harmonically related. Upstream transmission to the BIU can take place at a carrier frequency of 38.88 MHz, with a data rate of 19.44 Mb/s.
In an alternate embodiment, the downstream transmission takes place at a carrier frequency of 19.44 MHz, with a data rate of 51.84 Mb/s. This embodiment has the advantage of not disrupting the spectrum above 550 MHz, thus leaving all of that spectrum available for cable TV or other applications.
A method of digital downconversion is used for reception of upstream data in the BNU in which a baseband signal is generated by a downconversion process in which the first step is sampling of the RF passband signal at a rate which is equal to 4/3 the upstream carrier frequency. Subsequent to this sampling, a baseband signal is generated by multiplication of the sampled bandpass signal by exe2x88x92njxcfx80/2, where n represents a discrete time variable. The complex baseband signal is separated into a real and imaginary part to create the I and Q channels. The resulting baseband signals, one for the I channel and one for the Q channel, are interpolated to produce three output samples for every input sample. Low pass filtering is performed on the interpolated signals, and the resulting filtered baseband signals are decimated by eight to produce one output sample for every eight input samples.
An advantage of the present invention is that a minimum of phase-locked-loops are required, and that downconversion can be accomplished in a cost effective manner on a silicon integrated circuit, since many of the multiply operations in the digital signal processing steps can be eliminated.
These and other features and objects of the invention will be more fully understood from the following detailed description of the preferred embodiments which should be read in light of the accompanying drawings.